1. Field of the Invention
The present invention relates to a vacuum pump.
2. Description of the Related Art
A vacuum pump typified by a turbo-molecular pump is attached to a vacuum chamber of a dry etching apparatus or a CVD apparatus. A turbo-molecular pump is provided with a rotor which includes rotor blades and a rotor cylindrical section, stator blades which are arranged to face the rotor blades, and a screw stator which is arranged to face the rotor cylindrical section in the radial direction. The rotor rotates at high speed, specifically, at several tens of thousands revolutions per minute. The rotation of the rotor causes the rotor blades and the stator blades to cooperate with each other and causes the rotary cylindrical section and the screw stator to cooperate with each other. As a result, gas inside the vacuum chamber is discharged, which generates a high vacuum state inside the vacuum chamber.
In the above dry etching apparatus or CVD apparatus, there have been increasing cases in which a large amount of process gas is used to perform various kinds of processing. Even when a large amount of process gas is used, it is necessary to maintain a high vacuum state. Thus, a vacuum pump having a higher exhaust performance is desired. As a result, the number of stages in a vacuum pump tends to increase.
On the other hand, the height of the above dry etching apparatus or CVD apparatus is designed to allow an operator to easily perform an operation using the apparatus. A vacuum pump is typically attached to the bottom of the apparatus, and the height of the vacuum pump is thus restricted by the distance between the lower surface of the apparatus and a placement surface.
In order to design a vacuum pump to meet the above demands, an exhaust port may be arranged at a position facing a screw stator as described in Patent Literature 1 (Japanese Patent No. 4594689).
In such a case, an exhaust path for gas discharged by cooperation between a rotor cylindrical section and a screw stator leading to an exhaust port is narrowed, and the conductance of the exhaust path is thereby deteriorated. As a result, the exhaust performance of the vacuum pump is deteriorated.